The present invention relates to a filter for filtering electromagnetic interference coupled to and carried by power lines and, more particularly, to a filter for instantaneously filtering impulse or transient noise signals applied to a power line from such sources as lightning, nuclear explosions and large equipment switching surges which also continuously filters continuous type noise signals to protect sensitive electronic equipment connected to the power line.
Communications equipment, computers, military targeting systems, home stereo amplifiers, televisions and other electronic devices are increasingly characterized by small electrical contacts and miniature components which are very vulnerable to interference or damage from stray electrical energy of both the transient and continuous type carried by power lines. Unpredictable variations in power line voltage, as well as continuous noise thereon, changes the operating range of the equipment and can severely damage or destroy such electronic devices. These electronic devices are very expensive to repair or replace and, therefore, require cost effective transient and continuous type noise protection.
One source of harmful noise is radio frequency interference that can be coupled to the power lines from a multitude of sources. The power lines act as large antennas that can be stretched out over several miles and which can have significant radio frequency noise coupled thereto from such sources as radio broadcast antennas.
Another source of harmful radio frequency energy is from the equipment to be protected, such as computers. Today the Federal Communications Commission has elaborate test procedures to prevent computers from being marketed that cause severe noise problems. Nevertheless, older computers still exist which emit significant amounts of radio frequency interference.
Another harmful source is conducted noise generated by equipment connected to the power lines which is conducted along the power lines to the equipment to be protected.
A further source of harmful electrical energy is lightning. Lightning is a very complex electromagnetic energy source having potentials estimated at from five million to twenty million volts and currents reaching thousands of amperes. A lightning strike generally contains a series of pulses each having a duration of from one nanosecond to several milliseconds. A typical "8/20" lightning pulse lasts for a period of 40 milliseconds and has a peak current of 20,000 amps which is reached in 8 microseconds.
Another source of unwanted electromagnetic energy is a nuclear electromagnetic pulse. An electromagnetic pulse for a nuclear detonation produces intense transient electric and magnetic fields with very short rise times and a frequency spectrum extending from approximately zero to more than 100 megahertz. The electromagnetic pulse from a high altitude explosion typically has a maximum field strength near the ground on the order of 50 kilovolts per meter, a time duration of one microsecond and a rise time of nanoseconds.
Other sources of impulse noise are switchng transients caused by turning on and off large banks of equipment and large motors, and ground loop interference caused by varying ground potentials.
The above-mentioned noise transients and continuous noise can be coupled to a sensitive electronic device by external or internal coupling or induction. In external coupling, noise impinges on a receiver, such as a power transmission line or system, and the noise voltage induced in the receiver is passed through transformers, rectifiers and other voltage and current altering devices to the sensitive electronic device. The passage of the noise through such devices as transformers substantially alters the original waveform increasing the current levels, and the voltage levels, and altering the frequency spectrum. The characteristics of the noise reaching the sensitive equipment can be hard to predict because of the intermediate coupling devices, thereby making the noise hard to remove. Thus, there is a need in the art for a filtering device that has a broad range of energy absorption and/or shunting capability which responds continuously and instantaneously to any type of undesired noise.